Method and apparatus for developing electrostatic images with electroscopic powder



Nov. 6, 1951 L. E. WALKUP ETAL 2,573,881

METHOD AND APPARATUS FOR DEVELOPING ELECTROSTATIC IMAGES WITHELECTROSCOPIC POWDER Filed Nov. 2, 1948 INVENTORS Zbna HI. r

23.2) fiZar/zqy Patented Nov. 6, 1951 METHOD AND APPARATUS FOR DE-VELOPING ELECTROSTATIC IMAGES WITH ELECTROSCOPIC POWDER Lewis E. Walkupand Donald L. Fauser, Columbus, Ohio, assignors, by mesne assignments,to Battelle Development Corporation, Columbus, Ohio, a corporation ofDelaware Application November 2, 1948, Serial No. 57,962

7 Claims. (Cl. 101426) This invention relates to a method and apparatusfor developing electrostatic images with electroscopic powder, withreference particularly to the general type of apparatus in which aninsulating image layer on a, conductive plate is electrostaticallycharged and the image then developed by dusting thereon an electroscopicpowder which is subsequently transferred and aflixed to a suitabletransfer sheet or support, and it has for its purpose to afford aprocedure by which the background areas or non-image portions of theplate carrying the image layer are maintained substantially free fromelectroscopic powder, which adheres only to the image area, thusresulting in a copy or reproduction in which the background areas aswell as the image areas correspond closely to the original subject, soas to produce a faithful copy with background areas free fromundesirable shadows or darkened portions.

In the production of powder images on insulating image layers that havepreviously been electrostatically charged, it has been found that undesirable effects result in the background areas due to portions of theelectroscopic powder adhering to the non-image areas of the plate onwhich the image layer is supported, the powder that adheres to thenon-image areas being carried onto the transfer sheet along with thepowder on the image areas, and such powder as remains on the non-imageareas being subsequently charged and causing a build-up of powder and acontinual increasing of undesirable effects in the background areas inprinting operations or otherwise, where the image layer and supportingplate are repeatedly charged and dusted with electroscopic powder toproduce multiple copies, and it is a particular purpose of the inventionto afford a procedure and mechanism that will effectively preventadherence of the electroscopic powder to the non-image areas of theplate so that the latter is free from electroscopic powder except overthe image areas when the powder image is transferred.

The invention is particularly applicable to processes involving the useof developing materials consisting of a combination of finely dividedpigmented electroscopic powder with a coarser carrier material, thecarrier material being triboelectrically charged with a polarityopposite to that of the electroscopic powder upon frictional engagementtherewith and acting to retain the electrosccpic powder which isattracted to and surrounds the particles of carrier material, and if thecharge on the non-image areas of the plate has a greater attractiveforce for the electroscopic powder particles than the charge on thecarrier material has, the electroscopic powder particles are likely tobe attracted to and held by the non-image areas on the plate even thoughsuch non-image areas are of a conductive character to drain oil themajor part of the electrostatic charge except over the insulating imageareas, and it is a particular object of the invention to impose afurther electrostatic charge on the non-image areas of the plate or tosurround said areas with an electrostatic field such that the charge onthe non-image areas will have a polarity opposite to the charge on theimage areas and the same as the charge on the electroscopic powder, andwill therefore repel such powder and prevent its adhering to thenon-image areas.

A further purpose of the invention is to provide a practical,economical, and efiiclent construction in the form of an electrode orother electrostatic field-creating instrumentality that is adaptable toa continuous electro-printing machine or to other mechanism for applyingelectroscopic powder to a charged image layer, whereby the powder willbe applied evenly and uniformly over the image areas of the plate andthe non-image areas will be left substantially free of electroscopicpowder.

To these and other ends, the invention consists in the construction andprocedure that will appear clearly from the following description, whenread in conjunction with the accompany ing drawings, the novel featuresbeing pointed out in the claims following the specification.

In the drawings:

Fig. 1 is a diagrammatic sectional view illustrating one embodiment ofthe invention as incorporated in a continuous electro-printingapparatus, and

Fig. 2 is a diagrammatic view illustrating the action of an electrode increating an electrostatic field adjacent to and on a plate whenelectroscopic powder is applied to the latter.

The invention is applicable to the method disclosed in copendingapplication of Lewis E. Walkup and Edward N. Wise, Serial No. 24,674,filed May 1, 1948, and to the copending application of Roland M.Schafiert, Serial No. 55,816, filed October 21, 1948, which includes arotating drum on which is mounted a conductive printing plate carryingan insulating image layer, combined with means for applyingelectroscopic powder by gravity to the image layer as the drum rotates,after which the powder image is transferred to a continuous strip ofpaper or other material that 3 is continuously fed along in contact withthe drum at a point beyond the powder dusting area, the paper strip anddrum being constantly charged by suitable charging instrumentalitleswhich maintains an electrostatic charge constantly on the insulatingimage layer and also holds the paper strip in contact with the drum atthe point where the image is transferred to the paper by electrostaticaction.

Referring to Fig. 1 which discloses such a mechanism, I designates therotating drum whilq 2 is the conductive printing plate suitably mountedthereon and carrying the insulating image layer, which it is notnecessary to show for purposes of the present invention, while 3designates a discharge receptacle from which electroscopic powdergravitates over the inclined bottom 4 to fall onto the drum and traveldownwardly by gravity, the surplus electroscopic powder which is notretained by the charged image layer on the drum falling into thereceptacle 5. 8 designates the continuous paper strip that is fed from aroll I through a path adjacent to the drum'as shown and from which it isdeflected by the stripping fingers 8 after which the paper travelsdownwardly over an inclined chute or guide 9 onto a receiving roll H. I2 designates heating elements that act to fuse the powder image onto thepaper strip as it travels downwardly over the chute 9, while l3designates corona discharge needles or other suitable devices forapplying an electrostatic charge onto the paper strip 6 and onto theinsulating image of the drum. l4 designates any suitable cleaninginstrumentaliiw for removing from the drum surplus electroscopic powderafter each transfer operation, thus preparing the charged image layer toreceive a fresh supply of electroscopic powder and additional chargingunits such as shown at i and 16 may be employed if desired to imposeadditional charges on the insulating layer of the drum, although thesecharging units and their location form no part of the present inventionwhich will now be described in detail.

In order to prevent adherence of electroscopic powder to the non-imageareas of the printing plate, there is provided an electrode i!preferably curved as shown to conform to the curvature of the drum andlocated between the powder chamber wall 18 and the surface of the drum,being spaced at a distance of substantially 1.25" from the surface ofthe drum. The electrode ll is in the form of a plate having a lengthpreferably that of the drum I and extending around the drum from a pointimmediately beneath the inclined bottom 4 of the powder dischargereceptacle to a point below that at which the powder falls away from thedrum, so that the electrode substantially covers the entire area overwhich powder is in moving contact with the drum, and the electrode I!may consist of a solid -sheet, a screen, a series of \wires, or a seriesof points suspended or located over the area described, or near theplate surface during development, the electrode ll being connected byconductor IS with any suitable source of potential supply to create thedesired electake place and the effect produced by the electrode l1.Assuming that the insulating image area or layer 2|, carried by theconductive sup porting plate 22, is charged negatively, the electrode I1is likewise charged negatively by applying a suitable potential, andsuch negative charge on electrode l1 induces a positive charge asindicated on the non-image areas of the plate 22, shown at either sideof the image area 25. The electroscopic powder that produces the powderimage by adherence to the image area 25 is charged positively bytriboelectric action of the carrier material to which the electroscopicpowder particles adhere until separated therefrom by the strongerattractive force of the charged insulating image layer 2!. Since thenon-image areas of the plate are electrostatically charged positively bythe electrode H by induction, such positive charges repel the positivelycharged electroscopic powder particles, and thus any tendency of theelectroscopic powder to adhere to the non-image or background areas ofthe plate is effectively overcome.

The electrode ll having a charge of the same polarity as the insulatingimage layer exerts an attractive force on the electroscopic powder andto this extent may reduce the density of the powder image, but this isnot of appreciable significance when proper voltages are applied to theelectrode, and by suitably controlling the spacing between the electrodeand the insulating image layer and the potential applied to theelectrode, the electrostatic field between the electrode and theprinting plate can be adjusted as desired and the relative action of theelectroscopic powder on the insulating image layer and on the non-imageareas of the plate can be regulated and controlled to give the properand desired effect. By maintaining the polarity of the electrode chargethe same as that of the charge on the insulating layer and opposite tothe charge on the electroscopic powder, and properly controlling thespacing of the electrode from the image layer, also the potentialapplied to the electrode, undesirable or excessive adherence ofelectroscopic powder to the non-image or background areas of the platecan be definitely and efiectively prevented, and any desired relationbetween the action of the electroscopic powder on the image areas andnon-image or background areas can be maintained.

While the invention has been described with reference to the structureand procedure herein shown, the invention is not thus restricted andthis application is intended to cover such modifications or departuresas may come within the purposes of the improvements or the scope of thefollowing claims.

We claim:

1. The method of developing an electrostatic image on a chargedinsulating image layer supported on a conductive plate, and whichconductive plate extends across the background area oi the image, whichconsists in applying developing powder to the charged image layer whilemaintaining an electrical field, produced by a, potential having apolarity the same as that of the charge on the image layer and oppositeto that of the charge on the developing powder, over the area of theplate to induce a charge in the background area opposite in polarity tothe charge in the image area while the powder is applied, whereby Powderhaving the same charge as the background area is repelled from thebackground area.

' 2. The method 'of developing an electrostatical- 1y charged image onan insulating image layer supported on a conductive plate which extendsacross a background area, which consists in applying developing powderto the charged image layer and inducing on the non-image areas of theplate an electrical charge having a polarity opposite to that of thecharge on the image layer and the same as the charge on the developingpowder to repel powder from the background area, said last-mentionedcharge being produced by an electrical field applied to the plate whiledeveloping.

3. The method of developing an electrically charged image on aninsulating image layer supported on a conductive plate which forms thebackground area of an electro-printing plate consisting of applyingcharged developing powder to the charged plate, the powder being appliedbetween the plate and an electrode located adjacent to the plate, andapplying to said electrode a potential, having a polarity the same asthat of the charge on the image and opposite to that of the charge onthe developing powder, to induce a charge of the same polarity as thepolarity of the charge on the powder in the background area to repel thepowder from the background area.

4. The method of developing an electroprinting plate having anelectrostatic image on a charged insulating image supported by aconductive backing plate which forms the background area,.which consistsin applying charged developing powder to the surface of the plate onwhich the insulating image is charged oppositely to the developingpowder, the powder adhering to the image electrically, electricallyinducing a charge in the backing plate of the same polarity as thepolarity of the charge of the powder to repel the powder from thebackground area and thereby maintain the background area free of powder.

5. An electroprinting apparatus comprising in combination, a plateconsisting of an electrically conductive movable supporting backingplate carrying a nonconducting charged image layer, said backing plateextending across the background area of the image, means for chargingsaid image layer, powder-applying means for presenting to the platecharged developing powder having a polarity opposite to the polarity ofthe charged image layer, background charge inducing electrode mounted inspaced relation from the image layer in the image-powder attraction zoneto effect an electric field in said attraction zone to induce in thebackground area a charge of the same polarity as the polarity of thepowder. whereby said powder is simultaneously repelled from thebackground area and attracted to the image area.

6. An electroprinting mechanism for transferring a charged powder imageto a surface comprising an electroprinting plate consisting of anelectrically conducting movable support which forms the background areaof the printing plate having an image layer of electricallynonconducting material thereon, said image layer being chargedelectrically with a charge having a po-\ larity opposite to that of thepolarity of the charge of the powder to be transferred, powder-applyingmeans located adjacent to the movable support acting to flow a stream ofdeveloper powder charged with a charge of polarity opposite to that ofthe charge on the insulating image layer over the plate, said powderadhering to the image layer electrostatically to form a powder image, anelectrical field-producing electrode charged with the same polarity asthe image area mounted in spaced relation to the plate surface with thestream of powder flowing between the electrode and the plate, saidelectrical field being such as to effect an induced charge of the samepolarity as that of the powder in the background area to repel thepowder from the background area, and, electrostatic means fortransferring said charged powder image to a surface.

7. In an electric printing apparatus wherein moving support means areprovided to move a conductive printing plate which forms the backgroundarea of the plate having an insulating image on its printing surfacepast a charging station, electrical means at said station for placing acharge of a first polarity on said insulating image, powder-applyingmeans at a sequential station to deposit a powder carrying a charge ofsecond polarity on said image surface, the combination with saidpowder-applying means of, an electrode spaced from and overlying asubstantial part of the area over which said powder is deposited andmeans to charge said electrode to a potential of said first polarity toinduce a charge of said second polarity in the background areas of saidconductive printing plate during development thereof, said charge ofsecond polarity in the background area acting to repel the like chargedpowder from the background area.

LEWIS E. WALKUP. DONALD L. FAUSER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,221,776 Carlson Nov. 19, 19402,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 19442,359,476 Gravely Oct. 3, 1944 2,483,462 Huebner Oct. 4, 1949

